The present invention relates to a process for the preparation of isocyanates by purifying a product stream from the isocyanate synthesis.
Here, isocyanates are understood as meaning compounds having 1, 2 or more isocyanate groups (mono-, di- or polyisocyanates), preferably diisocyanates.
The novel process is suitable for all customary (cyclo)aliphatic and aromatic isocyanates or a mixture of two or more such isocyanates. Diisocyanates, for example monomeric methylene di(phenylisocyanate) (MDI), tolylene diisocyanate (TDI), R,S-1-phenylethyl isocyanate, 1-methyl-3-phenylpropyl isocyanate, naphthyl diisocyanate (NDI), n-pentyl isocyanate, 6-methyl-2-heptane isocyanate, cyclopentyl isocyanate, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 2,4 and 2,6-diisocyanatomethylcyclohexane (H6TDI) and the isomer mixtures thereof, o-, m- or p-xylene diisocyanate (XDI), diisocyanatocyclohexane (t-CHDI), di(isocyanatocyclohexyl)methane (H12MDI), tetramethyl-m-xylylene diisocyanate (m-TMXDI), 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI), diisocyanatocyclohexane (t-CHDI), 1,6-diisocyanato-2,2,4,4-tetramethylhexane, 1,6-diisocyanato-2,2,4-trimethylhexane and mixtures thereof (TMDI), are preferred.
The process for purifying TDI, monomeric MDI, HDI, IPDI, H6TDI, H12MDI, XDI, t-CHDI and NDI is particularly preferably used.
The literature describes various processes for the preparation of pure isocyanate.
U.S. Pat. No. 3,410,888 describes a process for isolating and purifying isocyanates. The process comprises the following steps: first, the reaction of a corresponding diamine with phosgene and the distillative separation of a part of the isocyanate thus prepared in the course of the solvent separation, secondly the transfer of the distillation residue (bottom product) into a second distillation apparatus (vessel), over the internal surface of which the residue is distributed as a thin film and the temperature and pressure of which are sufficient to effect vaporization of the isocyanate, and thirdly the removal of the vapor, which is substantially rich in isocyanate, from this second distillation apparatus.
The vapor is condensed and gives the isocyanate. Rising-film evaporators, falling-film evaporators and the like are mentioned as possible distillation apparatuses. The chosen solvent in the isocyanate synthesis usually has a lower boiling point than the isocyanate, preferably at least 30° C. lower. In the case of smaller boiling point differences, however, a part of the isocyanate prepared is separated off together with the solvent in the solvent separation. This is followed by the distillation of the crude isocyanate obtained as a residue, said distillation being effected in a thin-film evaporator. The partial isolation of the isocyanate in the solvent separation has the advantage that undesired medium boilers (possibly colored impurities or components whose boiling point is between that of the isocyanate and that of the solvent) are concomitantly separated off in the solvent separation. The mixture of the partly isolated isocyanate and the solvent is then recycled as a feedstock stream to the solvent separation, or it is fed to a separate evaporation or fractional distillation for concentrating the isocyanate. The latter is then recycled as a feed into the solvent separation.
The disadvantage of this process is that, owing to the evaporation in two steps in the case of isomeric isocyanates, e.g. TDI or MDI, fractions of different composition are obtained as a result of the different volatilities of the isomers. The purity of the isocyanates obtained moreover does not meet the present day requirements since low boilers are still contained. In addition, the product is lost via the high-boiling residue since the residue must be flowable in order to be capable of being transported out of the evaporator.
The prior German Application with the application number 10245584.8 and the date of filing of Sep. 27, 2002 describes a process for the preparation of isocyanates, the reaction discharge being present in the form of a suspension which contains the isocyanate to be prepared in the form of a liquid and carbamoyl chloride as a solid, in which process the suspension is worked up in a film evaporator. This film evaporator preferably has no moving parts, as, for example, a falling-film evaporator. The working-up can also be effected in a plurality of pressure stages in two or more film evaporators connected in series, the first film evaporator operating at a pressure of from 0.5 to 25 bar and the pressure of the second being from 0.01 to 1 bar lower than that of the first.
U.S. Pat. No. 5,962,728 describes the use of a thin-film evaporator in combination with a paddle dryer and a low boiler separation column. The crude isocyanate is fed to a thin-film evaporator. In the thin-film evaporator, the pure isocyanate is separated from the high-boiling, polymeric tar, the separation not being complete in order to obtain the tar in a form with sufficiently low viscosity. The tar stream still containing residues of the desired product is fed to a paddle dryer, in which the residual isocyanate is evaporated off from the tar. The isocyanate vapor, which still contains low-boiling impurities, is finally subjected to removal of the low boilers by distillation. The process is distinguished by the fact that the paddle dryer simultaneously has a heating zone and a cooling zone.
A disadvantage of the process is that the isocyanate stream leaving the arrangement is always taken off as bottom product and is therefore exposed to high thermal stresses, which leads to losses of NCO groups by oligomerization or polymerization of diisocyanate, since such high boilers are not separated off.
U.S. Pat. No. 3,140,305 describes the use of a horizontal thin-film evaporator for recovering aromatic diisocyanates. A disadvantage of the process is that the TDI recovered still contains low-boiling impurities, which complicates the direct use of the reaction discharge as a starting material in the polyurethane preparation. In addition, the desired product is lost from the high-boiling residue.
U.S. Pat. No. 4,216,063 describes the recovery of tolylene diisocyanate (TDI) in a thin-film evaporator at a wall temperature of from 210 to 250° C. and a pressure of from 1 to 50 mm Hg with a minimum residence time of 15 minutes. A disadvantage of the process is that the TDI recovered still contains low-boiling impurities, which complicates the direct use of the reaction discharge as a starting material in the polyurethane preparation.